In recent years, the development of X-ray CT apparatus that performs photon counting CT (computed tomography) using a photon counting type detector is progressed. Unlike an integration type detector used in conventional X-ray CT apparatuses, the photon counting type detector can count an incident X-ray photon individually, and outputs a signal capable of measuring (discriminating) energy of the incident X-ray photon.
Each of detection elements constituting the photon counting type detector outputs one signal (response waveform), for example, each time an X-ray photon enters. In photon counting CT, a response waveform output by the detection element is counted. Moreover, in the photon counting CT, the energy of an incident X-ray photon is measured from a signal intensity (height of wave, etc.) of the counted response waveform. This enables the photon counting CT to reconstruct an X-ray CT image with a high S/N (Signal per Noise) ratio, as compared with conventional X-ray CT images.
However, the response waveform takes time for attenuation, and therefore when X-ray photons enter at a high frequency (in high doses), a next photon enters before the signal, which is output by the previously incident photon, attenuates, and response waveforms of a plurality of X-ray photons overlap each other. Such a phenomenon is called “pile up”, and once a pile up occurs, the number of X-ray photons and their energy spectra cannot correctly be measured. Moreover, when an X-ray dose to be emitted is decreased so that X-ray photons enter at a low frequency (in low doses) to such an extent that the pile up will not occur, the number of photons counted by the photon counting type detector is reduced when a distance, in which the X-ray passes through a subject, is large, and the S/N ratio of a reconstructed image is degraded and an artifact occurs.
On the other hand, in the photon counting type detector, it is known that there is a detector capable of reducing occurrence of a pile up by providing a plurality of detection elements in an area corresponding to one pixel and adjusting the number of elements used for reconstruction according to an X-ray dose. However, the detector requires microfabrication (½, ⅓, or so) more than twice the detection element, and further requires a circuit for switching the detection elements, which leads to high production cost of the detector.